a) Field of the invention
The present invention relates to a zoom lens system which permits photographing an object located at an exceptionally short distance.
b) Description of the prior art
The photographic instruments, typically represented by cameras, are equipped with zoom lens systems. Though it is general to place great importance on operability and portability of the zoom lens systems, the fundamental importance of the zoom lens systems lies in the optical performance thereof. The zoom lens systems are prevailing especially for the convenience or versatility in practical use thereof. In the recent trend, the zoom lens systems are designed for wider vari-focal ranges, i.e., higher vari-focal ratios. At such high vari-focal ratios, it is difficult for the zoom lens systems to permit photographing objects located at shorter distances regardless of focusing method and the conventional zoom lens systems are equipped with an additional function for photographing objects located at the shorter distances in addition to the general functions of the zoom lens systems.
A certain type of the ordinary zoom lens system is focused on an object located at a short distance outside the ordinary focusing range by expanding the moving range of ordinary focusing lens unit. Another type of the ordinary zoom lens system is focused on an object located at a short distance outside the ordinary focusing range by moving lens unit other than the ordinary focusing lens unit.
All of these conventional zoom lens systems allow remarkable variations of aberrations to be caused by focusing and have a common defect that optical performance thereof is degraded for photographing objects located at the shorter distances. Especially curvature of field and distortion make insufficient or dull the image flatness, reproduction and marginal light amount for photographing or copying planar objects.
Since most of the conventional zoom lens systems are designed mainly for photographing objects located at nearly infinite distance and used for photographing objects located at the shorter distances as modifications, these zoom lens systems allow aberrations to be varied remarkably by moving the lens components or lens units for photographing objects located at short distances. It is of course possible to obtain favorable image quality for objects located at the shorter distances by designing a lens system like the macro lens systems, mainly for photographing objects located at short distances. In cases of the lens systems for cameras which are to be used for multiple purposes, however, such a design cannot provide sufficiently favorable image quality for an object located at a long distance.
In order to obtain favorable image qualities for objects located at long distance as well as the objects located at short distances, it is proper to combine a zooming system allowing little variations of aberrations to be caused by focusing and a focusing system allowing little variations of aberrations. However, it is difficult, by applying this concept to a zoom lens system having a high vari-focal ratio, to design a zoom lens system which can provide sufficiently favorable image qualities over the entire zooming range and over the entire focusing range.
On the other hand there is known a method to perform focusing especially by moving a lens component or a lens unit for photographing an object located at the shorter distance, and a telephoto zoom lens system for single reflex cameras designed on the basis of this method has already been developed. However, this zoom lens system is strictly limited in the photographing range thereof, has insufficient optical performance and is scarecely used in practice in the present days.